The invention relates to a process and an apparatus for the microsuspension polymerization of vinyl chloride in vessels whose contents are agitated predominantly by natural convection.
Polymerization reactors for polymerizing ethylenically unsaturated monomers in an aqueous medium, for example, for polymerizing vinyl chloride by a suspension, emulsion or microsuspension process, are predominantly constructed as vertically oriented pressure vessels with a stirrer and a cooling jacket. A pump is used to convey the cooling medium in the cooling jacket with the aim of increasing the flow velocity in the jacket and achieving a temperature which is, as far as possible, identical in all parts of the jacket. Dissipation of the heat of polymerization is improved by stirring the polymerization batch with a stirrer.
However, there are polymerization processes which have to be carried out without a stirrer because the aqueous polymerization mixture is sensitive to shear. An example of this is microsuspension polymerization of vinyl chloride homo- or copolymers, in which a homogenized aqueous dispersion of monomer, dispersing agent and free-radical generator is fed to the reactor and induced to polymerize. The homogenized dispersion cannot be stirred because it is sensitive to shear.
EP A 93936/(U.S. Pat. No. 4,528,337) describes an apparatus and a process for the microsuspension polymerization of vinyl chloride. The polymerization is carried out in a tubular shaped pressure vessel which has a large length to diameter ratio and is positioned vertically, optimizing heat exchange due to a large ratio of heat-exchange surface to contents and insuring agitation of the contents by natural convection, i.e., upwardly acting forces resulting from differences in temperature and/or density.
FIG. 1 depicts a reactor 1 of this type, representing the prior art. The reactor 1 is equipped with a cooling jacket 2 which serves as a heat exchanger to dissipate the heat of reaction. Water, as the cooling medium, is fed into a pumped circulation system 4 via a control valve 3, and, using a pump 5, is conveyed into the cooling jacket 2 via the inlet 6, which is arranged at the lower end of the reactor 1. The pump 5 is operated to insure a high circulation rate in order to insure uniform temperatures in the entire cooling jacket. The amount of water discharged at the overflow 7 at the upper end of the reactor 1 corresponds to the amount fed at the control valve 3. The flap valve 14 serves to direct the cooling water into the circulation system and prevents it from flowing out directly at the overflow 7.
At the start of the polymerization, the temperature in the water-filled pumped circulation system 4 is raised by feeding in steam via the control valve 8, in order to heat the reactor contents to polymerization temperature. After initiation of the exothermic polymerization reaction, the steam feed is stopped and the pumped circulation system 4 is cooled by feeding in cooling water via the control valve 3 in such a way that the heat of polymerization arising is dissipated to precisely the extent required to keep the temperature and/or the pressure of the reactor contents constant. To this end, pressure and temperature in the reactor 1 are determined with a thermometer 9 and a manometer 10; the thermometer 9 determines the inlet temperature of the cooling water, and after the measurements have been assessed, the appropriate amount of cooling water is introduced using the control valve 3.
The exothermic heat balance causes the polymerization batch to be more strongly heated in the middle of the reactor (in the vicinity of its axis) than in the vicinity of the walls. The resultant differences in density lead to an upward movement of the reactor contents in the vicinity of the axis and a downward movement in the cooled zone near the walls. This results in convective circulation, leading to agitation of the reactor contents together with improved heat transfer. In the embodiment described in EP-A 93936, this circulatory movement is supported by what is called "gas-bubble agitation", i.e., by blowing in an inert gas at the base of the reactor via the pipe 12 and, at the pipe 13, conducting away the inert gas which has been blown in.
A disadvantage of this method of operation is that the temperatures in the upper region of the reactor are markedly higher than in its lower region; these temperature differences become even greater as the polymerization progresses because the polymerization proceeds exothermically. A result of this is that the polymerization takes place at different temperatures and at different times in the regions mentioned, and therefore, there can be an undesirably broad distribution of properties decisive for quality such as the K value of the product.